The circulatory system is the means by which substances move rapidly to and from the interstitial fluid that bathes living cells in nearly all animals. Its smooth operation is absolutely central to maintaining operating conditions in the internal environment within a tolerable range- a state we call homeostasis.
I. Circulatory Systems-An Overview
a. General Characteristics
i. A circulatory system functions as an internal transport system of substances to and from cells.
ii. Three components that work together to maintain the volume, composition, and temperature of the interstitial fluid (the tissue fluid that bathes them) are:
1. Blood- a circulating connective tissue that interacts with the interstitial fluid and helps keep conditions tolerable for enzymes and other molecules that carry out cell activities.
2. Blood vessels- tubes, which differ in diameter and wall thickness, that blood is transported through.
3. Heart- a muscular pump that generates pressure to keep the blood flowing.
iii. Many animals have a closed circulatory system in which blood flow is confined to the heart and blood vessels that have continuously connected walls.
1. Blood volume moving through the system equals the volume returned to the heart.
2. The rate of blood flow slows down at capillary beds and in smaller-sized blood vessels called capillaries.
iv. Arthropods and most mollusks have an open circulatory system in which blood is pumped into tubes that open into a space in the body’s tissues.
v. After mingling with tissue fluids, the blood then moves into open-ended tubes, which lead back to the heart.
b. Evolution of Vertebrate Circulatory Systems
i. In fishes, blood flows in one circuit from back to the heart.
ii. Amphibians have a heart, which is partitioned into a right and left half so blood flow through two partially separated circuits.
iii. Birds and mammals have two separate circuits of blood flow to support the high level of activity typical of vertebrates on land.
1. In the pulmonary circuit, the right half of the heart receives deoxygenated blood and pumps it to the lungs for oxygenation.
2. In the systemic circuit, the left half pumps the freshly oxygenated blood throughout the body.
c. Links with the Lymphatic System
i. The lymphatic system picks up excess fluids, solutes, and disease agents from the interstitial fluid.
ii. Parts of the lymphatic system help cleanse bacteria and other pathogens from fluid from being returned to the blood.
II. Characteristics of Blood
a. Functions of the Blood
i. It carries oxygen, nutrients and other solutes to cells.
ii. It carries away their metabolic wastes and secretions, including hormones.
iii. It stabilizes internal pH.
iv. It serves as a highway for phagocytic cells that scavenge tissue debris and fight infections.
v. In birds and mammals, it helps equalize body temperature.
b. Blood Volume and Composition
i. Blood volume of an averaged-sized adult is about 6 to 8 percent of the total body weight, or 4 to 5 quarts.
ii. Blood is made up of plasma, red blood cells, white blood cells and platelets.
1. Plasma is the fluid portion of blood that is made mostly of water.
a. It functions as a transport medium for blood cells and platelets, a solvent for ions and molecules, a defense against pathogens, a lipid transporter, and it has roles in extra cellular fluid volume and pH.
b. It contains ions, glucose, lipids, amino acids, vitamins, hormones, and dissolved gases.
2. Red Blood Cells (Erythrocytes)
a. They are biconcave and contain hemoglobin, an iron-containing protein that binds with oxygen.
b. They transport oxygen used in aerobic respiration and they carry away some carbon dioxide wastes.
c. They form in the red bone marrow from stem cells.
d. When they mature, they have no nuclei; they live about 120 days.
e. Cell count remains at 5.4 million microliter for males and 4.8 for females.
3. White Blood Cells (Leukocytes)
a. They remove dead cells and protect us against invading viruses, bacteria and other invaders.
b. They arise from stem cells in the bone marrow.
c. They differ in size, nuclear shape and staining traits. There are five categories:
i. Neutrophils are “search and destroy” cells.
iv. Monocytes are “search and destroy” that develop into macrophages.
v. Lymphocytes, the “B” and “T” cells, are involved in the immune responses.
4. Platelets are fragments of megakaryocytes produced by bone marrow stem cells without a nucleus.
a. They release substances that initiate blood clotting.
b. They last only 5 to 9 days, but hundreds of thousands are always circulating.
III. Human Cardiovascular System
a. “Cardiovascular” comes from the Greek words meaning “heart” “vessel.”
i. Blood travels from the heart à arteries à arterioles à capillariesà venules à veins àheart.
ii. The human cardiovascular system consists of two separate circuits, pulmonary and systemic, for blood flow.
1. The pulmonary circuit, a short loop, rapidly oxygenates blood. It leads from the heart’s right half to capillary beds in both lungs, and then returns to the heart’s left half.
2. The systemic circuit is a long loop starting at the heart’s left half. Its main artery, the aorta, accepts the oxygenated blood à arterioles à capillary beds à veins à heart’s right half.
IV. The Heart is a Lonely Pumper
a. Heart Structure
i. The heart is a durable pump, made mostly of cardiac muscle and in enclosed in a tough, fibrous sac. It has chambers lines with connective tissue and endothelium.
ii. The oxygen-demanding cardiac muscle cells have their own coronary circulation; coronary arteries branching off the aorta lead to capillary bed that services only them.
iii. Each half of the heart consists of an atrium chamber, which receives, and a ventricle chamber, which pumps, separated by an atrioventricular (AV valve).
iv. Blood leaves through semilunar valve.
b. Cardiac Cycle
i. Each time the heart beats; the chambers go through a sequence of contraction (systole) and relaxation (diastole), called the cardiac cycle.
ii. When relaxed, the aorta is filled with blood.
iii. An increase in blood pressure forces the AV valve to open; the ventricles continue to fill as the atria contract. When the ventricles contract, the Av valve closes and blood flows out through the semilunar valves.
c. Mechanisms of Contraction
i. In cardiac muscle tissue, the ends of cardiac muscle cells branch then connect with one another.
ii. With each heartbeat, the signals spread so fast that cardiac muscle cells contract together, almost as if it were a single unit.
iii. The SA node fires action potentials faster than the rest of the system and serves as the cardiac pacemaker, the basis of a normal rate of heartbeat.
V. Blood Pressure in the Cardiovascular System
a. Two factors influence the rate of flow through each type of blood vessel:
i. The flow rate is directly proportional to the pressure gradient between the start and end of the vessel.
ii. The flow rate is inversely proportional to the vessel’s resistance to flow.
b. Blood pressure is fluid pressure imparted to blood by heart contractions.
c. Arterial Blood Pressure
i. Arteries have a large diameter of a thick, muscular, elastic wall and present low resistance to flow; they serve as rapid transporters of oxygenated blood and are pressure reservoirs that smooth out pulsations caused by each cardiac cycle.
ii. Systolic pressure is the peak pressure that the contracting ventricles exert against the artery’s wall during a cardiac cycle.
iii. Diastolic pressure is the lowest pressure when blood is draining into the vessels after it.
d. Resistance to Flow at Arterioles
i. Vasodilation is when control signals cause smooth muscle cells to relax.
ii. Vasoconstriction is when other signals cause the smooth muscle cells to contract, decreasing the diameter of the blood vessel.
e. Controlling Mean Arterial Blood Pressure
i. Cardiac output is influenced by controls over the rate and strength of heartbeats, and total resistance mainly by vasoconstriction at the arterioles.
ii. Baroreceptor reflex is the main short-term control over arterial pressure.
iii. Long-term control of blood pressure is exerted at kidneys, which adjust the volume and composition of the blood.
VI. From Capillary Beds Back to the Heart
a. Capillary Function
i. Capillary beds are diffusion zones for exchanges between blood and interstitial fluid.
1. The capillary is so small that red blood cells travel in a single file when passing though it.
2. A capillary is a tube of single layer of endothelial cells, which facilitates diffusion.
ii. Movement across the capillary is done by diffusion, endocytosis, exocytosis, and by bulk flow.
1. At the start of a capillary bed is a movement of plasma out of the interstitial fluid. This process is called ultrafiltration.
2. Reabsorbption is when the inward-directed force exceeds the outward force of blood pressure, tissue fluid moves through the clefts between endothelial cells and into the capillary.
b. Venous Pressure
i. Capillaries merge into venules (“little veins”) and then merge into veins.
ii. Veins are large-diameter, low-resistance transport tubes to the heart that have valves to prevent backflow.
iii. A vein wall contains some smooth muscle that helps blood circulate faster during physical exercise. Also, skeletal muscles bulge against veins that help raise the venous pressure, driving blood back to the heart.
a. Hemostasis, a process involving blood vessel spasm, platelet plug formation, and blood coagulation, may repair the damage and stop blood loss.
b. The repair process includes blood vessel spasm, platlet plug formation, and coagulation.
VIII. Lymphatic System
a. Lymph Vascular System
i. The lymph vascular system is a portion of the lymphatic system that consists of many tubes that collect and transport water and solutes from interstitial fluid to ducts of the circulatory system, mainly lymph capillaries and lymph vessels.
ii. The lymph vascular system serves three functions:
1. Its vessels are drainage channels for water and plasma proteins that have leaked away from the blood at capillary beds and must be delivered back to the blood circulation.
2. It takes up fats that the body has absorbed from the small intestine and delivers them to the blood circulation.
3. It delivers pathogens, foreign cells and material, and cellular debris from the body’s tissue to be disposed at the lymph nodes.
iii. The lymph vascular system begins at the capillary beds, then they merge into lymph vessels, and then converge into collecting ducts that drain into veins in the lower neck.
b. Lymphoid Organs and Tissues
i. Lymphatic organs and tissues have roles in defending the body against damage and attack; it includes lymph nodes, the spleen, the thymus, tonsils and patches of tissue in the small intestine and appendix.
ii. Lymph nodes are located at intervals along lymph vessels.
iii. The spleen, the largest lymphoid organ, filters pathogens and used-up blood cells from the blood.
iv. The thymus gland is the sight where certain white blood cells acquire means to chemically recognize specific foreign invaders.